T50

• What is the difference between T50 and T40?
• What is the difference between T50 and T50 Air?
• What is the difference between T50 and T80?
• Is the headrest removable?
• How do I assemble the backrest and the seat pan?
• Even if I lock the tilt in the most upright position, it doesn't stop at 90 degrees. Is it defective?
• I want to fix the backrest at a certain angle. Do you have any suggestions?
• How can I fix the seat slope at a certain angle?
• Do you have casters for the hard floors? If you don't, what is the dimension of the casters?
• There is a line back of the seat. Is it defective?